Method of fabricating porous silicon carbide (SiC)

ABSTRACT

Porous silicon carbide is fabricated according to techniques which result in a significant portion of nanocrystallites within the material in a sub 10 nanometer regime. There is described techniques for passivating porous silicon carbide which result in the fabrication of optoelectronic devices which exhibit brighter blue luminescence and exhibit improved qualities. Based on certain of the techniques described porous silicon carbide is used as a sacrificial layer for the patterning of silicon carbide. Porous silicon carbide is then removed from the bulk substrate by oxidation and other methods. The techniques described employ a two-step process which is used to pattern bulk silicon carbide where selected areas of the wafer are then made porous and then the porous layer is subsequently removed. The process to form porous silicon carbide exhibits dopant selectivity and a two-step etching procedure is implemented for silicon carbide multilayers.

This invention is the subject matter of a NASA contract, contract No.NAS-3-26599 and the U.S. Government may have rights thereunder.

This application is a continuation-in-part of application Ser. No.07/957,519 filed Oct. 6, 1992, now U.S. Pat. No. 5,298,767.

RELATED APPLICATIONS

The assignee herein, Kulite Semiconductors Products is the record ownerof U.S. Patent application entitled "HIGH TEMPERATURE TRANSDUCERS ANDMETHODS OF FABRICATING THE SAME EMPLOYING SILICON CARBIDE", Ser. No.07/694,490, filed or May 2, 1991 for Anthony D. Kurtz et al, now U.S.Pat. No. 5,165,283. See also U.S. patent application 07/777,157 filed onOct. 16, 1991 entitled "METHOD FOR ETCHING OF SILICON CARBIDESEMICONDUCTOR USING SELECTIVE ETCHING OF DIFFERENT CONDUCTIVITY TYPES"by J. S. Shor et al., now abandoned and assigned to Kulite.

FIELD OF THE INVENTION

This invention relates to semiconductor devices in general and moreparticularly, to semiconductor devices which employ single crystalsilicon carbide (SiC) and methods of making porous silicon carbide.

BACKGROUND OF THE INVENTION

In the recent literature, there has been much interest inelectrochemical processes which cause semiconductors, such as silicon,to become porous. Various articles have appeared in Applied PhysicsLetters and other publications relating to such devices. See forexample, an article by V. Lehman and U. Gosele, Applied Physics Letters,Volume 58, Page 856 (1991). See also an article in Volume 57 of AppliedPhysics Letters, by L. T. Canham, Page 1046 (1990). The prior art wascognizant of the fact that in certain instances, porous silicon exhibitsunique properties which are superior to those of bulk silicon. Forexample, high efficiency luminescence has been observed in poroussilicon above the 1.1 eV band-gap of bulk material, which suggests thatoptical devices can be fabricated based on the use of porous silicon.Control of the pore size on the nanometer scale can allow porousmaterials to be used as filters in solid state chemical sensors.

In any event, there are several theories for the formation mechanisms ofpores in silicon. A good reference is the article by R. L. Smith and S.D. Collins appearing in the Journal of Applied Physics, Volume 8, R1(1992). Studies suggest that the depletion regions of pores overlap,causing a carrier depletion in the interpore region, and thus thecurrent is confined to the pore tips. In an article that appeared in theJournal of the Electrochemical Society, Volume 138, Page 3750 (1991) byX. G. Zhang, there was indicated that pore propagation is attributed toa higher electric field at the pore tips which causes dissolution tooccur more rapidly through the intermediate step of silicon dioxideformation, while along the pore walls dissolution occurs through theslower process of direct dissolution.

Demonstrations of room temperature visible luminescence from poroussilicon have generated much interest in using the material foroptoelectronics. Initially, there was of course much conjecture aboutthe mechanisms which provide the visible luminescence. However, ageneral consensus has been reached among most researchers that at leasta portion of the luminescence is associated with quantum structures(wires or dots) in the porous silicon. These quantum structures wouldallow a relaxation of the momentum selection rules by confining thecharges spatially, thus allowing direct band-gap transitions.Additionally, the charge confinement would increase the effectiveband-gap, thereby pushing it into the visible region. The quantumconfinement theory has been supported by considerable theoretical andexperimental evidence.

Researchers, such as C. Tsai, K. H. Li and D. S. Kinosky, et al., in anarticle in Applied Physics Letters, Volume 60, Page 1770 (1992) haveshown that surface chemistry, specifically hydrogen termination, play animportant role in the luminescence. This suggests that luminescence inporous silicon may have similar mechanisms as a-Si, which exhibits bandgap widening into the visible region when hydride species are formed onthe surface. A portion of the visible luminescence of porous Si may beassociated with the SiH. It has yet to be conclusively determinedwhether the hydrogen termination serves only to passivate the surface orwhether there is a contribution to the luminescence by amorphous Sihydride. Nevertheless, it is very clear that microcrystals of<5 nmdimension will exhibit band-gap widening and above band-gapluminescence.

There has been interest in SiC as a semiconductor material since the1950's. Its wide band-gap, high thermal conductivity, high breakdownelectric field and high melting point make SiC an excellent material forhigh temperature and high power applications. SiC also exhibitsinteresting optical properties, such as deep UV absorption, visibletransparency and blue photo- and electro-luminescence. However, goodquality crystals were unavailable, causing the early research efforts tostagnate. Recent developments in single crystal epilayer and boulegrowth have generated new interest in SiC. This has resulted in thedevelopment of SiC blue LED's, UV photodiodes and high temperatureelectronic components. However, due to its indirect band-gap, theefficiency of SiC optoelectronic devices is limited. Thus, research isunderway to develop other wide band-gap semiconductors, such as SiC_(x)AlN_(y) and III-V nitrides for optical applications. However, thecrystal growth technology for these materials is still veryunderdeveloped. Porous SiC could be very useful, since it has superioroptical properties than SiC, and may benefit from the relatively maturegrowth and processing technology that SiC has to offer. Devices whichwould benefit from these superior optical qualities include LED's,Lasers, and Photodetectors. Furthermore, SiC is very difficult to etchbecause of its chemical inertness. Therefore, porous SiC could also beused to pattern this material for electronic device fabrication.

There have been several reports on the electrochemical dissolution ofSiC. Recently, Shor, et al. used laser assisted electrochemical etchingto rapidly etch high relief structures in SiC. See an article by J. S.Shor et al., in Journal of Electrochemical Society, Volume 139, Page143, May 22, 1992. M. M. Carrabba et al., in an article inElectrochemical Society Extended Abstracts, Volume 89-2, Page 727(1989), reported etching diffraction gratings in n-type β- and α-SiC atanodic potentials with a uniform light source. The fundamentalelectrochemical studies indicate that the presence of HF in aqueoussolutions is important to etch SiC electrochemically. Glerria andMereming in Volume 65 of the Journal of Electroanl Chem., on Page 163(1975) reported that α-SiC dissolves in aqueo US H₂ SO₄ solutions atanodic potentials through the formation of a passivating layer, whichwas suggested to be SiO₂, since it dissolved in HF.

The present invention relates to the formation of porous SiC, a newmaterial. It is indicated that porous SiC material itself, as well as aprocess to fabricate the porous SiC is provided. Porous SiC can beemployed for UV and blue light sources such as LED's and diode-lasers.Porous SiC can be utilized as a filter in chemical processes and can beused to provide heterojunction devices using the porous SiC/bulk SiCinterface. As will be described, the methods employ a selective etchingof bulk SiC by forming a porous layer on the surface, oxidizing it andstripping it in hydrofluoric (HF) acid. One can also provide dielectricisolation of SiC devices on a wafer by oxidizing a buried porous SiClayer.

SUMMARY OF THE INVENTION

This invention relates to porous SiC, its fabrication, and utilizationin semiconductor devices. Three applications of porous SiC are disclosedin this light.

1) Porous SiC can be fabricated in a manner which will result in asignificant portion of nanocrystallites within the material in the sub10-nm regime. This will result in bandgap widening and a much moreefficient luminescence from the material. As such, porous SiC hassuperior properties than bulk SiC for semiconductor light sources, suchas LEDs and Lasers. Thee use of porous SiC in these devices enable themto operate at UV wavelengths. When passivated, porous SiC exhibits amuch brighter blue luminescence (by a factor of 50) than bulk material,enabling more efficient blue light sources. The enhanced properties ofporous SiC are also useful for photodetectors.

2) Porous SiC also is useful as a sacrificial layer for the patterningof SiC. SiC is a very inert material, and as such, is difficult to etchby conventional methods. Porous SiC, however, can be removed from itsbulk substrate by oxidation and other methods. Therefore, a two stepprocess can be used to pattern bulk SiC, whereby selected areas of thewafer are made porous, and then the porous layer is subsequentlyremoved. The process to form porous SiC exhibits dopant selectivity(i.e. one conductivity type becomes porous while another is unaffected).Thus, using the two step etching procedure, dopant selective etch-stopsmay be implemented for SiC multilayers.

3) Porous SiC can be oxidized at a much faster rate (several orders ofmagnitude) than bulk SiC. This property can be utilized to fabricatedielectrically isolated SiC layers and/or selectively introduced thicksections of SiO₂ into a SiC wafer. SiC-on-insulator is fabricated byoxidizing a buried porous layer which is underneath a non-porous SiClayer. The porous layer is oxidized completely, and the remainingstructure is dielectrically isolated SiC. This aspect of porous SiC isvery useful in the fabrication of high temperatures/power/frequencyelectronic components.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram depicting a process for forming porous SiC utilizingan electrochemical cell.

FIG. 2 is a top plan view transmission electron micrograph of a porousSiC layer formed by the process described in FIG. 1.

FIG. 3 is a top plan view electron micrograph of a pattern etched into alayer of SiC.

FIGS. 4-7 depict various steps in employing SiC to form a dielectricallyisolated SiC device (e.g. the diode) according to this invention.

DETAILED DESCRIPTION OF THE INVENTION

This descriptio is divide into four parts for clarity: 1) thefabrication method of porous SiC, 2) the use of porous SiC foroptoelectronic devices, 3) the use of porous SiC for patterning orshaping SiC and 4) the use of porous SiC to achieve dielectricallyisolated SiC layers and devices.

Fabrication of Porous SiC

Referring to FIG. 1, there is shown apparatus which can be used to formporous SiC. The formation of porous SiC occurs under electrochemicalanodization. There are wide variety of fabrication conditions thatresult in pore formation, and the microstructure, pore size, porespacing and morphology of the material is dependent on the processparameters. Referring to FIG. 1, there is shown an electrochemical cell22. The cell 22 may be fabricated from an electrolyte-resistant,dielectric material, such as Teflon plastic material (trademark ofdupont Company). The cell 22 has a lead 27 which is a platinum wirecounter-electrode and a lead 28 which is a saturated calomel referenceelectrode. Both leads are directed to a control processor apparatus 30to control the entire process as will be explained. The cell 22 containsa electrolyte 35. The electrolyte 35 used in cell 22 may be ahydrofluoric acid (HF) solution which is relatively dilute, as forexample a 2.5% HF or any other acidic solution containing F⁻ or Br⁻ ionsotherwise capable of dissolving SiO₂. A semiconductor wafer such asn-type 6H-SiC samples are contacted electrically with nickel ohmiccontacts and are placed in a carrier 24 which carrier is as indicatedplaced within the electrochemical cell 22. The semiconductor sample isencapsulated in black wax so that only the bare semiconductor surfacesare exposed as the ohmic contacts and the leads are protected. Thus asseen in FIG. 1, the semiconductor wafers are positioned and mounted inthe carrier module 24 which is positioned on the top surface of apedestal 26 which is located in the cell 22. The semiconductor acts asthe working electrode in this arrangement. Care must be taken, that allsurfaces not to be etched, which may corrode in the electrolyte, must becovered with the black wax or other encapsulant. The semiconductor SiCis preferably biased with respect to the saturated calomel referenceelectrode 28 at a suitable potential for the n-type layer to photocorrode. The bias voltage is provided to the control processor 30. Inthis embodiment an anodic potential is applied to the semiconductor. Inthe case of n-type SiC, ultraviolet or UV light from source 20illuminates the sample surface in order for dissolution to occur. Thelight is directed through a sapphire window 21 where it impinges uponthe surface of the semiconductor supported by the carrier 24. In p-SiC,dissolution can occur in the dark. The depth of the porous layer and itsstructure (e.g. pore size and interpore spacing) is determined by theanodization time, the UV light intensity, the applied potential, the pHand the doping levels of the crystals. As such the porosity, interporespacings, and morphology can be controlled by varying these parametersproperly. As indicated, the semiconductor is subject to electrochemicaletching in the electrolyte 35, preferably while being exposed to UVlight. UV light is provided from the UV light source 20 through thelight transmissive cover 21 sealed to the top of the cell 22 by means ofseals 23. The UV exposure generates holes in the semiconductor in thearea which is exposed by the ultraviolet light. In any event, for a moredetailed description of anodic dissolution of SiC, see the above-notedapplication which was filed on Oct. 16, 1991 entitled METHODS FORETCHING OF SILICON CARBIDE SEMICONDUCTOR USING SELECTIVE ETCHING OFDIFFERENT CONDUCTIVITY TYPES, Ser. No. 07/777,157. Again refer, ring toFIG. 1, it is indicated that pore formation will occur under thefollowing process conditions. For n-type 6H-SiC the anode potential isequal to 0-2 V_(sce) (sce=saturated calomel electrode) which is appliedby the control processor 30. The UV intensity from light sourede 20 ismaintained between 50-500 mW/cm². The UV wavelength is selected in therange of 250-400 nanometers. The carrier concentration of the siliconcarbide wafer is 3×10¹⁸ /cm³. The concentration of the HF solution 35 isequal to 2.5% in water. In order to obtain porous p-type silicon forp-type 6H-SiC, the anodic potential is equal to 1.8 to 2.8 V_(sce) thecarrier concentration is equal to 2-3×10¹⁸ /cm³ and the HF concentrationis equal to 2.5% in water. The conditions for porous film formation arenot limited to those delineated above, but pore formation has beendirectly observed under the above conditions. Under some of theseconditions, pores spacings which are below 10 nm will result.

It should be noted that the potential of pore formation in n-SiC islower than that of p-SiC. Therefore a porous layer can be formed on a pnjunction, such that the n-SiC side of the junction becomes porous andthe p-SiC is unaffected. This can also be accomplished in reverse byanodizing the p-SiC between 1.8-2.8V_(sce) in the absence of UV, suchthat the p-SiC becomes porous and the n-SiC is unaffected.

Referring to FIG. 2, there is shown a top plan view transmissionelectron micrograph of a porous SiC layer formed in a n-type 6H-SiC atV=1.4 V_(sce), I=300 mW/cm² of UV (250-400 nm), and N_(d=) 3×10¹⁸ cm⁻³in 2.5% HF. The bright areas 40 are voids or pores in the SiC. The poresmay range in size between 10 and 100 nm. The spacing between these poresrange between about 5 nm to 100 nm. This indicates that both quantumcrystalline structures (less than 10 nm) and enlarged crystallinestructures can be fabricated. The pore size, shapes and spacings arevery much a function of the processing conditions. Electron diffractionof the porous areas prove that the material is single crystal 6H-SiC.Thus, as one can ascertain from the micrograph of FIG. 2, one canproduce porous SiC as disclosed therein and according to the above-notedprocess. In any event, porous SiC has the potential to be utilized as aUV generation medium in light emitting diodes (LED's) and laser diodes.Such devices would be extremely useful in optical storage,optoelectronic communication systems, laser bumping systems,sensor/detectors and materials processing. These devices may emit lightin the UV wavelength.

Optoelectronics from Porous SiC

Semiconductor optoelectronics has, by and large, been limited to III-Vcompounds due to their direct band-gap. For SiC, a porous structurecould increase its already wide band-gap (3 eV for 6H-SiC) and allowdirect gap transitions, facilitating which efficient UV/near UVluminescence. These luminescent properties are useful in LED or a Laser,thus greatly enhancing current optoelectronic capabilities by includingdeeper wavelengths in semiconductor light sources. These direct bandgaptransitions also enable more efficient blue LEDs to be fabricated whenporous SiC is used rather than bulk SiC.

SiC has unique optical properties, such as blue electro-luminescence,which have facilitated the development of blue LED's. However, due tothe indirect band-gap of SiC (3 eV for 6H-SiC), the LED's areinefficient. By electrochemically fabricating a microcrystalline porousstructure in SiC, it is possible to increase both the band-gap andquantum efficiency, resulting in UV, or deep Blue luminescence. Thisluminescence will enable the development of semiconductor UV and bluelight sources and UV/blue optoelectronic devices from porous SiC.

As noted earlier, porous microcrystalline SiC structures can beelectrochemically fabricated with pore spacings of "quantum" dimensions(less than 10 nm) in accordance with the present invention. Suchstructures exhibit luminescence above the band gap. Moreover, theluminescence in the blue range of the spectrum (approximately 2.8 eV) isgreatly enhanced by passivating such structures with a passivating agentsuch as oxygen or hydrogen. Passivation enables the microcrystallinestructures to satisfy the conditions for quantum confinement bypreventing surface recombination at dangling bond. Passivating agentsthat may be employed for this purpose include atomic hydrogen, depositedby a plasma or by a HF dip, oxygen, formed by thermal oxidation oranodically, or any other passivating agent which will pin the danglingbond sites. The enhanced luminescence can be utilized in the fabricationof a variety of optoelectronic devices such as blue semiconductor lightsources (e.g., light emitting diodes) and semiconductor lasers. Itshould also be noted that there are a wide variety of conditions whichwill result in this superior form of porous silicon carbide.Essentially, any thin insulating layer deposited or grown on the poroussurface, could provide the desired passivation layer. Thus, theresulting thin passivation layer may be comprised of SiN, SiH, SiO_(n),or the like.

Laser diodes and LED's have been used extensively in a wide diversity ofapplications ranging from displays to optical communication systems.Porous SiC UV sources will extend the wavelength capability of thisdevice below the blue wavelengths currently available from singlecrystal SiC and ZnSe. Such light sources could be useful in a variety ofapplications. For example, UV sources would enable a smaller spatialvolume in optical recording, thus enhancing both spatial resolution andinformation packing densities in optical storage. UV LED's and lasersare also useful for optical communications and as higher energy pumpsources for LASER's and possibly phosphors.

Currently, efforts are underway to develop direct band gap materialswith large band gaps, such as GaN, AlN and ZnSe, which have band gapsbetween 3.2-6.4 eV for UV and near UV optoelectronics. Amicrocrystalline SiC structure would be useful in applications (e.g.blue and UV optoelectronics) for which the other wide gap materials arebeing investigated. SiC has much more sophisticated device technologyassociated with it than these other materials and thus offersconsiderable advantages over these other materials.

Porous SiC is also useful for photodetectors. Porous SiC has a very lowreflectivity compared to bulk SIC, which allow more of the incidentradiation to be collected. The wider bandgap of the porous SiC enablesthe easy fabrication of heterojunction photodetectors, which are know toexhibit superior properties than homojunction detectors.

Patterning of SiC using Porous SiC

SiC is a very difficult material to pattern into device structuresbecause of its chemical inertness. By selectively fabricating a porouslayer into a silicon wafer, oxidizing the layer and removing the oxidein HF, deep etched features can be patterned.

Referring to FIG. 3, there is shown a SEM micrograph of a pattern etchedinto n-type 6H-SiC, by the intermediate step of forming a porous layer.A metal mask was deposited on the SiC provided as described above, andpatterned using standard photolithographic processes. The etchingconditions to form porous SiC were I=500 mW/cm² of UV (250-400 nm),V=1.5 V_(sce), 2.5% HF for 30 minutes. The mask was subsequently removedand the SiC was thermally oxidized in a steam ambient for 4 hrs. at1150° C. to fully oxidize the porous layer, and form a thin oxide (<1000Å) on the parts of the surface previously covered by the metal. Theoxide was etched for 2 min in buffer HF, resulting in the pattern 45formed on the SiC. These conditions are an example of how SiC can beetched by the two step process of 1) forming a porous layer on thesurface and 2) removing the porous layer. Furthermore, by employing theselective anodization conditions discussed earlier, a layer of oneconductivity type can be etched, while a second layer of a differentconductivity type acts as an etch-stop.

Dielectric Isolation Using Porous SiC

The following steps exhibit how porous SiC can be used to dielectricallyisolate SiC devices. In this case the device described is a pn-junctiondiode, but the principles are equally applicable to other devices aswell. Referring to FIG. 4, one proceeds with an n-SiC wafer 50, twoepilayers, one p-type 51 and one n-type 52 are grown by chemical vapordeposition. In FIG. 5a, a mask 54 in placed on the top n-type epilayer52 and defined using photolithography. In FIG. 5b, the n-SiC layer 52 isetched by the previously mentioned methods. In FIG. 6, the p-SiC layer51 is patterned in a similar manner. In FIG. 7, the bottom n-SiC 52becomes porous and p-SiC 51 remains inert. The top n-SiC epilayer doesnot come into contact with the solution. The porous layer is thenoxidized, resulting in a dielectrically isolated pn-junction. Thepn-junction is comprised of single crystal SiC layers (which are notporous).

As one can ascertain, the techniques described above can be implementedby many different procedures as briefly alluded to. Reference is againmade to U.S. patent application 07/694,490 entitled HIGH TEMPERATURETRANSDUCER AND METHODS OF FABRICATING THE SAME EMPLOYING SILICONCARBIDE, filed on May 2, 1992 and assigned to assignee herein. In thatapplication, a pertinent reference was cited and entitled GROWTH ANDCHARACTERIZATION OF CUBIC SiC SINGLE CRYSTAL FILMS ON SILICON by J.Anthony Powell et al., published in the Journal of ElectrochemicalSociety, Solid States Science and Technology, June 1987, Volume 134, No.6, Pages 1558-1565. This article contains an extensive bibliography andapproaches using SiC in various applications and in the processing ofSiC. As is well known, one can grow silicon carbide of either n orp-type by means of chemical vapor deposition techniques (CVD). Suchtechniques are well known for growing silicon carbide on silicon wafers,see for example the above-noted article. The doped gas employstri-methyl aluminum. In a similar manner, the growth of silicon carbidelayers on n-type silicon carbide is also known and can be accomplishedby conventional CVD techniques. There are various articles in the priorart which teach the growth of films of silicon carbides. See for examplean article by J. A. Powell, L. G. Matus and M. A. Kuczmarsld in theJournal of the Electrochemical Society, Volume 134, Page 1558 (1987).See also an article by L. G. Matus, J. A. Powell, C. S. Salupo, AppliedPhysics Letters, Volume 59, Page 1770 (1991). Such articles, as well asthe above-noted applications, teach the growth of layers of siliconcarbide utilizing either p or n-type silicon carbide on silicon wafersor wafers of silicon carbide. In any event, such techniques employstandard masking techniques as for example, standard photolithographicprocesses which are also well known in the art.

Thus there is described at least one useful device utilizing porous SiC,but many other devices are in fact contemplated. There is described theuse of porous SiC to pattern SiC. There is described a method of formingSiC-on-insulator structures using porous SiC. There is described amethod and technique for the formation of porous SiC, a material thatcan have wide spread utility. The material as well as the process, willenable the formation of various devices with great potential use.

What is claimed is:
 1. A method of fabricating porous SiC comprising thesteps of:placing a wafer of silicon carbide in a electrochemical cell;electrochemically etching said wafer for a period sufficient to formpores on a exposed surface of said wafer; and illuminating said exposedsurface of said wafer with ultraviolet (UV) light.
 2. The methodaccording to claim 1, wherein said wafer is n-type SiC.
 3. The methodaccording to claim 2, wherein said n-type SiC has a carrierconcentration of about 3×10¹⁸ /cm³ with said SiC being 6H-SiC.
 4. Themethod according to claim 1 wherein said wafer is p-type SiC.
 5. Themethod according to claim 4, wherein said p-type SiC has a carrierconcentration of between 2 to 3×10¹⁸ /cm³.
 6. The method according toclaim 5, wherein said step of electrochemically etching includesapplying an anodic potential of from 2 to 3 V at a saturated referenceelectrode for 1 to 60 minutes.
 7. The method according to claim 1,wherein the intensity of said UV light is between 50-500 mW/cm³ and thewavelength is between 250-400 nanometers.
 8. The method according toclaim 1, wherein said step of electrochemically etching includesapplying an anodic potential to said wafer in said cell.
 9. The methodaccording to claim 8, wherein said potential is between 0-3 volts. 10.The method according to claim 1, wherein said etching employshydrofluoric acid (HF).
 11. The method according to claim 10, whereinsaid HF is 2.5% in water.
 12. The method according to claim 1, furtherincluding oxidizing porous portions of said wafer after said etchingstep to convert said porous silicon carbide into silicon dioxide. 13.The method according to claim 12, further including densifying theregions of silicon dioxide.
 14. The method according to claim 13,wherein said densifying step comprises annealing said regions of silicondioxide.
 15. The method according to claim 1, wherein pores of submicrondimensions are formed during said etching step.
 16. The method accordingto claim 1, wherein a monocrystalline layer of porous silicon carbide isformed during said etching step.
 17. The method according to claim 1,wherein said pores spacings are less than 10 nm.
 18. The methodaccording to claim 1, further including the step of passivating asurface of said layer.
 19. The method according to claim 18, whereinsaid passivating step comprises forming a thin insulating layer on thesurface of the said porous layer.
 20. A process for etching siliconcarbide comprising:providing a substrate or epilayer of silicon carbide,said silicon carbide being of a first conductivity type; selectivelyanodizing said substrate electrochemically to form porous regions ofsilicon carbide thereon; and removing said porous regions from saidsubstrate.
 21. The process according to claim 20, further including thestep of oxidizing said porous regions prior to said removing step. 22.An etch stop process comprising the steps of:providing a SiC layer of afirst conductivity type, on top of a layer of SiC of a secondconductivity type; and anodizing the first layer so that the secondlayer remains inert, to form porous SiC in place of the first layer; andremoving the porous layer.
 23. The process according to claim 22,wherein said porous regions are defined photolithographically prior tosaid anodizing step.
 24. The process to claim 2 for providing aheterojunction of porous SiC and non-porous SiC.
 25. A method offabricating porous SiC comprising the steps of:placing a wafer ofsilicon carbide in a electrochemical cell; electrochemically etchingsaid wafer for a period sufficient to form pores on a exposed surface ofsaid wafer wherein said pores define spacings of less than 1 micron. 26.The method according to claim 25 further including the stepof:illuminating said exposed surface of said wafer with ultraviolet (UV)light.
 27. The method according to claim 25, wherein said step ofelectrochemically etching includes applying an anodic potential to saidwafer in said cell.
 28. The method according to claim 27, wherein saidpotential is between 0-3 volts.
 29. The method according to claim 25,further including oxidizing porous portions of said wafer after saidetching step to convert said porous silicon carbide into silicondioxide.
 30. The method according to claim 29, further includingdensifying the regions of silicon dioxide.
 31. The method according toclaim 30, wherein said densifying step comprises annealing said regionsof silicon dioxide.
 32. The method according to claim 25, wherein amonocrystalline layer of porous silicon carbid, is formed during saidetching step.
 33. The method according to claim 25, further includingthe step of passivating a surface of said layer.
 34. The methodaccording to claim 33, wherein said passivating step comprises forming athin insulating layer on the surface of the said porous layer.